The field of the invention is magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to magnetic resonance elastography (“MRE”) inversion methods and systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It has been found that MR imaging can be enhanced when an oscillating stress is applied to the object being imaged in a method called MR elastography (“MRE”). The method uses the oscillating stress to produce shear waves that propagate through the organ, or tissues, to be imaged. These shear waves alter the phase of the MR signals, and from this the material properties of the subject can be determined. In many applications, the production of shear waves in the tissues is a matter of physically vibrating the surface of the subject with a device referred to as an “MRE driver.” Shear waves may also be produced, for example, in the breast and prostate by direct contact with the oscillatory device. Also, with organs like the liver, the oscillatory force can be directly applied by means of an applicator that is inserted into the organ.
It has been suggested that cardiac dysfunction is related to the mechanical properties of the myocardium and that knowledge of these parameters could provide insight into a variety of diseases. Exemplary diseases include diastolic dysfunction, hypertension, and myocardial ischemia. To date, however, the application of MRE to quantify myocardial tissue mechanical properties has provided inaccurate results.
Like the heart and many organs in the body, disease states of the eye, such as macular degeneration, myopia, and cancer are also often indicated by changes in the mechanical properties of its constituent tissues. Assessments of ocular, intraocular, and orbital rigidity, however, are currently limited to qualitative assessment by direct palpation, more invasive methods, or other conventional methods such as tonometry. Yet, these methods may yield indirect or inaccurate results. As with the heart, to date, MRE has yet to be applied to determine the material properties of the eye in an accurate manner.
It would therefore be desirable to have a system and method for non-invasively analyzing the mechanical properties of organs such as the heart and eye.